In recent years, the conventional spool type ink ribbons used in impact printers are being increasingly substituted by cassette type ink ribbons. A cassette type ink ribbon is folded to be contained in the narrow space of a cassette. When the cassette type ink ribbon is used for printing by a printer, the characters printed at the folds become disadvantageously darker than those printed at the other portions of the ink ribbon. The more darkly printed spots are called graphic spots. In recent years, new types of printers quite free from the graphic spots such as laser printers, heat transfer printers and ink jet printers are being marketed, and in this connection, there is an increasing demand for base fabrics for ink ribbons not allowing the graphic spots to occur. To meet the demand, a technique for lessening the graphic spots by disordering the array of monofilaments by high pressure fluid treatment to enhance the ink absorbability is proposed in Japanese Patent Laid-Open No. 91-281277, and techniques for lessening the graphic spots by specifying the weave are proposed in Japanese Patent Laid-Open Nos. 91-34883 and 91-73378.
However, the high pressure fluid treatment proposed as a means for enhancing the ink absorbability of the base fabric requires expensive and additional equipment investment and increases the number of production steps, to considerably raise the cost. The techniques for specifying the weave of the base fabric cannot be said to be satisfactory in the effect to lessen the graphic spot's, and since the weave is loose, the fabric is liable to be caught by the head pins of a serial dot printer as a problem in view of practical use.